This research applies the synthetic model compound approach to cytochrome oxidase in order to deduce molecular level structure at the active site. Using tetraphenylporphyrin derivatives it is planned to synthesize iron (III) porphryrins which are covalently linked to copper (II) complexes. Once definitively characterized, preferably by Xray crystallography, the magnetic and structural properties will be investigated by EPR, MCD, Mossbauer, EXAFS and susceptibility studies as a function of bridging ligand (oxo, hydroxo, cyanide, imidazolate, carboxylate, etc.), spin state at iron, iron-copper distance and magnetic orbital orientation. Derivatives of such complexes will be studied as analogues of the cytochrome oxidase intermediates in the catalytic reduction of dioxygen to water. Iron(IV) oxo species of heme a will be prepared to test various proposals for such states in cytochrome oxidase. The H-bonded dioxygen moiety in oxygemoglobin will also be modelled as part of an investigation of dinuclear versus protic trapping of dioxygen at the active site.